Prestressed concrete is a type of reinforced concrete slab which has been subjected to an external compressive force prior to the application of loads. Prestressed concrete is categorized as either pre-tension or post-tension. Pre-tension refers to the method of first stressing tendons and then casting concrete around the prestressed tendons. The concrete cures before releasing the prestressed tendons and transferring the stress from the tendons to the concrete. A tendon comprises prestressing assemblage including steel systems, anchor mechanisms, coatings and sheathings.
Post-tension refers to the method of casting wet concrete around unstressed tendons and then stressing the tendons after the concrete has reached specified strength. Post-tension concrete is generally categorized further as being either bonded or unbonded. Post-tension concrete which is bonded refers to the tendons generally being covered with a duct or pipe or conduit such that after the concrete around the duct, for example, has set and cured, the tendon is stressed and a grout material generally consisting of cement, sand, aggregate and water is pumped into the cavity surrounding the wire system so that the space between the walls of the conduit and the steel is completely encased in grout. Unbonded post-tension concrete refers to the use of greased or paper wrapped or plastic covered wire systems around which the concrete is cast. The unbonded system is permanently without bond between the steel and the concrete because of the grease, paper or plastic.
Although the modern method of prestressing concrete may be traced to the late 1920's, its general use in the United States did not begin until the late 1940's or early 1950's. General acceptance and the primary increase in use occurred primarily between 1965 and 1975. Application of prestressing was being made in all aspects of construction including buildings, towers, floating terminals, ocean structures and ships, storage tanks, nuclear containment vessels, bridge piers, bridge decks, foundations, soil anchors, and virtually all other types of installations where normal reinforced concrete was acceptable. Thus, prestressed concrete and methods for its initial installation for diverse applications is now well known.
After years of service, however, problems associated with prestressed concrete structures became apparent. The problems appeared primarily on consideration of future use of structures and/or maintenance of deteriorated or damaged structures.
In the 1960's and 1970's, it was common to build multistoried parking ramp structures for automobiles. About that same period, public utilities began relying on the application of calcium chloride or sodium chloride to remove or reduce the effect of ice and snow on bridges and roads. In addition, automobiles day after day and year after year would carry the calcium chloride or sodium chloride into the parking structures. It is now known that the chloride ion leaches or otherwise moves into concrete slabs. In contact with reinforcing steel, the chloride ion has caused considerable corrosion. It has not mattered whether the steel was normal reinforcing steel or steel used for prestressing tendons. Bridges and parking structures estimated to have useful maintenance free lives of 10 to 25 years are now requiring maintenance in only 7 to 15 years.
Thus, as corrosion of the prestressing steel and the normal reinforcing steel became apparent, it became necessary to provide for methods of maintenance, repair, and replacement of distressed decks and floors. The accepted method for repair became total slab removal and replacement. Partial removal and replacement of a concrete slab containing tensioned tendons was generally not possible. In most cases, the tensioned concrete deck or slab was two directional having tensioned tendons running at right angles to one another. Removal of concrete in one direction allowed deformation in the other direction. Even if the slab was generally one directional, the use of secondary tendons for shrinkage control was widely accepted and therefore tendons still ran at right angles to primary reinforcement. Thus, the prestressing concept, widely accepted a decade or two ago, is now shocking structure owners as complete slab replacement becomes necessary, even though only two to four percent of total slab surface area is damaged. Even more frightening, however, are the consequences to the general public if a structure owner in view of repair costs performs only cosmetic repair and allows the strength of slabs to deteriorate to the point of collapse.